Rewrite varnish.py to use the varnishstat JSON interface

varnishstat in varnish 3.0 has a JSON interface that is easier
to parse and provides more info.

The plugin can now also write its own config file,
which is useful as it allows new metrics to be detected
and added on the fly.

Because the varnish instance (hostname if empty) is now
always prepended to the metric name, this change is
backwards incompatible.

Change-Id: I2e44affa6d6f5726407879c08574b21b802a9d25